This application is based on Japanese Patent Applications Nos. 2000-274761 and 2001-102921 filed respectively on Sep. 11, 2000 and Apr. 2, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an image display apparatus that is used in front of a viewer""s face and that presents the viewer with a displayed image along with the image of the outside world.
2. Description of the Prior Art
In recent years, image display apparatus that are used in front of a viewer""s face and that direct both the light from a displayed image and the light from the outside world to the viewer""s eyes so as to present the viewer with the displayed image along with the image of the outside world have been proposed as a personal data presentation apparatus. In such image display apparatus, to secure a wide visual field, a display section for displaying an image is disposed in a peripheral portion of the apparatus, and an eyepiece optical system for directing the light from the display section to the eyes and a transparent plate for holding the eyepiece optical system are disposed in front of the eyes. As the eyepiece optical system is used, for example, a reflective hologram element that reflects the light from the display section and that transmits the light from the outside world so that the image of the outside world is presented to the viewer with an enlarged virtual image of the displayed image superimposed thereon.
This type of image display apparatus is typically shaped like spectacles so that its user (the viewer) can use it in every situation in his or her daily life. For example, the user can observe displayed images while walking.
The hologram element, used as the eyepiece optical system, is affixed on the surface of the transparent plate, or embedded in the transparent plate. In the latter case, the transparent plate is produced by cementing together two component pieces having slant end surfaces, and the hologram element is affixed beforehand on the cementing surface between those two component pieces. In this arrangement, the light from the display section is introduced into the transparent plate through an edge portion thereof, and is then reflected between the opposing surfaces of the transparent plate so as to be directed to the hologram element.
However, in conventional image display apparatus, the hologram element, used as the eyepiece optical system, is so shaped as to have an edge surface substantially perpendicular to the surfaces of the transparent plate, i.e. with almost no angular difference with respect to the viewer""s line of sight. As a result, some external light is reflected from the edge surface of the hologram element and enters the eyes. Similarly, some external light is reflected from the edge surface of the transparent plate and enters the eyes. Moreover, in an arrangement in which the transparent plate is produced by cementing together two component pieces, some external light is reflected from their cementing surface and enters the eyes. Such unwanted light, which would not enter the eyes if no reflection took place, enters the eyes from directions different from the directions from which it is supposed to, and thus degrades the quality of the image presented to the viewer.
Moreover, as shown in a top view in FIG. 27, in an arrangement in which the transparent plate is produced by fitting together two component pieces 13a and 13b with the gap 13g between them filled with adhesive 32 so that the two component pieces 13a and 13b are cemented together, the adhesive 32 may contract (sink in) or expand (swell out). Such contraction or expansion of the adhesive 32 refracts or diffracts the light LB from the outside world, and thereby cause unwanted light, i.e. light that is usually not supposed to enter the viewer""s eyes E, to enter the eyes E, degrading the quality of the image presented. This problem arises not only with adhesive 32, but also in an arrangement in which the gap between the two component pieces 13a and 13b or a slit formed in the transparent plate is filled with a photosensitive material to form a hologram element, in which case the contraction or expansion of that material causes similar unwanted refraction and diffraction.
Although a hologram element is an optical element that offers excellent characteristics, using only one hologram element as an eyepiece optical system as in conventional image display apparatus is insufficient to achieve further enhancement of the performance of the eyepiece optical system beyond a certain limit.
An object of the present invention is to provide an image display apparatus that operates with minimum degradation of image quality due to unwanted external light, and to provide an image display apparatus furnished with an eyepiece optical system that offers excellent performance.
To achieve the above object, according to one aspect of the present invention, in an image display apparatus that is provided with a display section for displaying an image, an eyepiece optical system, and a transparent plate-shaped member for holding the eyepiece optical system, that is used with the eyepiece optical system placed in front of a viewer""s eyes and with the plate-shaped member facing the eyes, and that is designed to direct light from the display section through the eyepiece optical system to the eyes so that the viewer is presented with a virtual image of the image displayed on the display section and simultaneously permit light from the outside world to be transmitted through the plate-shaped member so as to be directed to the eyes so that the viewer is presented with an image of the outside world, the boundary surface between the eyepiece optical system and the plate-shaped member is so arranged as not to reflect light from the outside world incident thereon at angles of incidence greater than the Brewster angle toward the eyes.
In general, when light is incident on the interface between two media having different refractive indices, the reflectance depends on the angle of incidence. FIGS. 26A and 26B show the relationship between the angle of incidence and the reflectance as observed in a case where the two media respectively have refractive indices of 1 and 1.5. FIG. 26A shows the reflectivity observed when light travels from the medium having the lower refractive index to the medium having the higher refractive index, and FIG. 26B shows the reflectivity observed when light travels from the medium having the higher refractive index to the medium having the lower refractive index. The angle of incidence at which the reflectivity Rp toward P-polarized light equals 0 (indicated by arrows BA) is the Brewster angle, and, when the angle of incidence with respect to the interface equals the Brewster angle, the reflected light travels perpendicularly to the transmitted light.
When the angle of incidence is smaller than the Brewster angle, both the reflectivity Rp toward P-polarized light and the reflectivity Rs toward S-polarized light are low, and thus most of the incident light is transmitted through the interface. By contrast, when the angle of incidence exceeds the Brewster angle, both the reflectivity Rp toward P-polarized light and the reflectivity Rs toward S-polarized light abruptly become high, causing most of the incident light to be reflected from the interface.
Accordingly, by designing the interface between the eyepiece optical system and the plate-shaped member not to reflect toward the eyes the light from the outside world that is incident at angles of incidence greater than the Brewster angle, it is possible to greatly reduce the amount of light from the outside world that is reflected from that interface so as to enter the eyes. In this way, it is possible to avoid degradation of the quality of the image presented to the viewer.
According to another aspect of the present invention, in an image display apparatus that is provided with a display section for displaying an image, an eyepiece optical system, and a transparent plate-shaped member for holding the eyepiece optical system, that is used with the eyepiece optical system placed in front of a viewer""s eyes and with the plate-shaped member facing the eyes, and that is designed to direct light from the display section through the eyepiece optical system to the eyes so that the viewer is presented with a virtual image of the image displayed on the display section and simultaneously permit light from the outside world to be transmitted through the plate-shaped member so as to be directed to the eyes so that the viewer is presented with an image of the outside world, the eyepiece optical system is provided with two or more hologram elements, of which at least the last-stage one is held by the plate-shaped member.
According to another aspect of the present invention, in an image display apparatus that is provided with a display section for displaying an image, an eyepiece optical system, and a transparent plate-shaped member for holding the eyepiece optical system, that is used with the eyepiece optical system placed in front of a viewer""s eyes and with the plate-shaped member facing the eyes, and that is designed to direct light from the display section through the eyepiece optical system to the eyes so that the viewer is presented with a virtual image of the image displayed on the display section and simultaneously permit light from the outside world to be transmitted through the plate-shaped member so as to be directed to the eyes so that the viewer is presented with an image of the outside world, a coating portion is provided to coat a plurality of members exposed on the surface of the plate-shaped member facing the viewer""s eyes.